introduce tuples to tidy up the code

[codecov[bot]]

Codecov Report

Base: 98.83% // Head: 98.87% // Increases project coverage by +0.04% :tada:

Coverage data is based on head (b6e94e0) compared to base (6966bde). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #261 +/- ## ========================================== + Coverage 98.83% 98.87% +0.04% ========================================== Files 22 22 Lines 7445 7272 -173 ========================================== - Hits 7358 7190 -168 + Misses 66 62 -4 + Partials 21 20 -1 ``` | [Impacted Files](https://codecov.io/gh/ichiban/prolog/pull/261?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase) | Coverage Δ | | |---|---|---| | [engine/builtin.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2J1aWx0aW4uZ28=) | `99.09% <100.00%> (-0.02%)` | :arrow_down: | | [engine/clause.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2NsYXVzZS5nbw==) | `98.58% <100.00%> (-0.03%)` | :arrow_down: | | [engine/compound.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2NvbXBvdW5kLmdv) | `100.00% <100.00%> (ø)` | | | [engine/exception.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL2V4Y2VwdGlvbi5nbw==) | `100.00% <100.00%> (ø)` | | | [engine/number.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL251bWJlci5nbw==) | `100.00% <100.00%> (ø)` | | | [engine/parser.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3BhcnNlci5nbw==) | `99.85% <100.00%> (-0.01%)` | :arrow_down: | | [engine/term.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3Rlcm0uZ28=) | `100.00% <100.00%> (ø)` | | | [engine/vm.go](https://codecov.io/gh/ichiban/prolog/pull/261/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase#diff-ZW5naW5lL3ZtLmdv) | `87.98% <100.00%> (+1.03%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yutaka+Ichibangase)

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